In the prior art, it is known to deposit diamond films of adequate quality for many commercial applications. An article in Science magazine, volume 234, Nov. 18, 1986, pages 1074-1076 describes the state of the art at that time in the development of diamond deposition technology. The article describes the work of an American group at Pennsylvania State University which has deposited polycrystalline diamond films from a mixture of hydrogen and methane on silicon substrates with the aid of a microwave plasma. Apparatus developed in Japan is also discussed in which a tungsten filament is heated to 2000.degree. C. to excite hydrogen and methane just above a silicon, silica, or molybdenum substrate on which diamond is to be deposited. The mixture of pyrolized methane and atomic hydrogen flows toward the substrate and carbon atoms from the pyrolyzed methane are deposited on the heated substrate to form diamond crystals. Microwave and radio frequency plasmas have also been used as an energy source to dissociate the hydrogen and excite the methane.
In a publication in June, 1986 of the Nippon Institute of Technology, the then current state of the art is described as including the use of the tungsten filament method with silicon substrates and the use of organic hydrocarbon gasses containing oxygen or nitrogen. An example is described wherein acetone is used and polycrystalline films are stated to have grown at a rate of 10 micrometers per hour.
The article also discloses a growth method which is termed remote plasma-enhanced chemical vapor deposition in which a molecular gas is dissociated in a location away from the substrate. The resulting monomers and organic gas then flow through a heated region toward the substrate where pyrolysis and deposition are said to occur.
The above cited Science magazine article further states that W. G. Eversole of Union Carbide obtained a patent in 1958 for method of obtaining diamond films from pyrolysis of methane. The process was not commercially attractive because substantial amounts of graphite were deposited along with the diamond so it was necessary to interrupt the growth process periodically to etch away the graphite. A group of scientists at the Institute of Physical Chemistry in Moscow published a paper in 1977 concerning the kinetics of the pyrolysis of hydrocarbons which discussed the concept of a solvent which could prevent the deposition of graphite while not affecting the deposition of diamond. Atomic hydrogen was suggested as the solvent, the same substance used by Eversole as the etchant in his process.
An article in the Oct. 26, 1987 edition of Electronic Engineering Times, describes a technique that Fujitsu Ltd. claimed to have developed to produce diamond films by projecting a high density DC plasma against a substrate.
The book High Temperature Vapors by John W. Hastie describes a vapor phase/surface reaction mechanism and method of characterizing the total volitization rate from a surface due to evaporation and surface reactions. It further discusses the thermodynamic prediction and experimental verification of gas transport of carbon from a cold to a hot or a hot to a cold surface.
A paper entitled "Hot Filament For Diamond Growth" by Fang and Rhais describes the use of a carbon hot filament in an electromagnetic deposition device. The choice of carbon was made primarily to avoid certain physical metallurgical deficiencies encountered with the more typically used tungsten. The graphite is selected for its presumed stability.
It has recently been shown that diamond can be deposited when the jet from an oxy-acetylene torch is directed toward a cooled substrate, provided the ratio of oxygen and acetylene and the gap between the torch nozzle and the substrate are closely controlled. (See L. M. Hanssen, et al., "Diamond Synthesis Using An Oxygen Acetylene Torch".)
The above techniques for depositing a diamond film all have serious disadvantages. None of them are suitable for depositing diamond films over large areas or over large curved surfaces. Few of them hold any real promise for being sufficiently efficient to make them commercially feasible in most applications. In most instances, the energy and materials costs required to practice the techniques cause the diamond film produced therefrom to be far more expensive than the cost of commercial diamonds as mined or as produced through high pressure and high temperature techniques.